Radio receiving and transmitting circuitry often uses filter circuitry to decrease the level of unwanted signals in relation to a desired signal. Resonant circuits containing combinations of inductors, capacitors and resistors may be used to provide such filter circuitry. Such circuits are known as LC (for inductor-capacitor circuits) or RLC (for resistor-inductor-capacitor circuits).
On example of the use of an LC circuit as a filter will be described below. In this example the filter is arranged between a signal line and a line carrying a fixed voltage, typically a ground or supply voltage—VDD—line. The LC circuit comprises a capacitor and an inductor in parallel. The impedance of this parallel LC circuit across most of the frequency spectrum is relatively low. However in a certain range near to the circuit's resonant frequency, the impedance of the parallel LC circuit is relatively high.
In operation, the parallel LC circuit will attenuate signals on the signal line in dependence on the parallel LC circuit's impedance at the frequency of the signals. Signals at a frequency where the LC circuit has a relatively low impedance will be attenuated; that is, these signals will experience a large reduction in signal level. This is because these signals will be connected to ground/VDD by a low impedance connection, and thus will be grounded or shorted out. By contrast, signals at a frequency where the LC circuit has a relatively high impedance, will not be attenuated as much; that is these signals will have only a small reduction in signal level. This is because such signals will be relatively isolated from the ground/VDD line.
Such an LC circuit provides a simple band-pass filter, because only ‘in-band’ signals—those at, or near, the LC circuit's resonant frequency—will pass without significant attenuation. By contrast, out-of-band signals will be attenuated.
In some situations, the degree of filtering provided by the filter circuitry may be insufficient. In other words the attenuation of the out-of-band signals will be insufficient to prevent detrimental performance of the system in which the filter is embedded. This is a particular, and increasing, problem in both transmitters and receivers, because regulations limiting out of band transmissions are becoming stricter and the increase in radio transmissions from other sources has increased the quantity of interference experienced by a typical receiver.
Embodiments described herein are intended to address these problems.